The government traditionally blames extremists for the violence. Uighur activists, on the other hand, point to ethnic tensions and tight Chinese control as triggers for violence.

Uighurs and Xinjiang

Uighurs are ethnically Turkic Muslims

They make up about 45% of the region's population; 40% are Han Chinese

China re-established control in 1949 after crushing short-lived state of East Turkestan

Since then, large-scale immigration of Han Chinese

Uighurs fear erosion of traditional culture

Verifying reports from the region is difficult because the information flow out of Xinjiang is tightly controlled.

News of the latest clash first emerged on the state-run regional Tianshan news portal.

Officials described the people who attacked the police station in Yarkand, near the old silk road city of Kashgar, as "thugs carrying knives and throwing explosives".

It was not immediately clear if there were any police casualties. The incident is currently under investigation, officials say.

More than 100 people have been killed in Xinjiang this year in this and similar incidents, which Beijing blames on separatist "terrorists" from the Uighur group, says the BBC's John Sudworth in Shanghai.

In late October, five people were killed when a car ploughed into a crowd and then burst into flames in Beijing's Tiananmen Square.

Beijing called the incident a terrorist attack inspired by Xinjiang-linked extremists. Three people who died inside the car were identified by police as Xinjiang Uighurs.